Phosphorescent materials having light emitting and charge transport properties are increasingly desirable for organic electronic and opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic thin film transistors (OTFT), organic chemical sensors, organic photovoltaic cells (solar cells), and organic photo detectors.
Of the variety of electronic and opto-electronic applications of phosphorescent materials, organic light emitting diodes (OLED) are the most attractive for flat display and general lighting industry. OLEDs make use of thin organic films that emit light when voltage is applied across the device. For OLED applications, phosphorescent materials are potentially more attractive over fluorescent materials as the former may make use of triplet emission and yield nearly 100% internal quantum efficiency [M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and S. F. Forrest, Applied Physics Letter., 1999, 75, 4]. Most of phosphorescent materials investigated for OLED devices are based on phenyl-pyridine ligand [Takao Takiguchi, et al., US2002064681]. The most popular one is tris(2-phenylpyridine)iridium (Irppy), which has been successfully employed in fabrication of green OLED devices. However, this is not the case for other colors from phosphorescent materials based on phenylpyridine ligand. The well-known blue phosphorescent material based on phenylpyridine ligand is iridium(III)bis(2-(4,6-difluorophenyl)pyridinato)picolinate (Firpic) [S. Lamanski, et al, US2002182441], which suffers from a short life-span and low efficiencies.
Therefore, the need for new phosphorescent materials are highly desirable for the commercialization of the organic electronics, OLED in particularly. In search for high performance phosphorescent materials, researchers around the world have explored variety of iridium complexes based on ligands such as benzoquinoline (bzq), phenylbenzothiazole (bt), naphthylbenzothiazole (bsn), phenylquinoline (pq), thienylpyridine (thp), benzothienylpyridine (btp), phenyloxazole (op), diphenyloxazole (dpo), phenyl-isoquinoline (piq) and many others. Although significant progresses have being made though various structure modifications, there is still a need for better materials with novel structures.
It is well know that oxadiazole derivatives have appreciable electron transport properties, and they have been successfully employed in many OLED device configurations [Saito Shogo, et al., JP04363894]. Using oxadiazole derivatives to make metal complexes is expected to improve charge transfer between ligand and metal, ultimately enhance the performance of the opto-electronic devices. However, oxadiazole based compounds have not been employed to make metal complexes for phosphorescent materials.